1. Field of the invention.
This invention relates to circuits for controlling operation of internal combustion engine ignition systems that utilize magnetos and, in particular, non-incendive shut-down systems for such engines. The terms "incendive" and "non-incendive" as used herein mean having and not having, respectively, enough electrical energy to ignite; see National Electrical Code, 1975, published by National Fire Protection Association, page 70-352, paragraph 501-3. (b) (1) (c).
2. Prior Art
Stationary internal combustion engines operating unattended for long periods of time frequently are equipped with automatic shut-down devices to protect the engine from expensive damage should serious malfunctions, such as low lube oil pressure, high jacketwater temperature, or excessive vibration, occur. Companion to the shut-down system is an annuciator panel which shows which of the primary sensing elements (low oil switch, etc.) caused the engine to shut down. This "indicator" panel is useful to the operator or service man in pinpointing the malfunction and simplifies the problem of repair. Conventional electronic shut-down panels draw their operating power from the engine magneto system and upon actuation shut the engine down by grounding the magneto primary circuit. When this circuit is grounded through ordinary contacts arcing can occur with sufficient energy release to ignite flammable mixtures of gases or dust.
The present invention discloses a means for substituting non-arcing, solid state switches in those portions of the circuit handling incendive levels of electrical energy and a means for reducing to non-incendive levels the electrical energy that must be handled by ordinary contacts in the circuit. Arranging the circuit in this manner permits the use of ordinary contacts and general purpose enclosures in National Electrical Code Division 2 areas where, otherwise, special contacts would be required as, for example, contacts which are hermetically sealed or immersed in oil or contained in explosion-proof enclosures. Special contacts and explosion-proof enclosures would always be more costly than the system proposed by this invention; and in some cases, suitable equipment that would provide a degree of safety equivalent to that of the system of the present invention is not available.